This invention relates generally to the field of closures for metallic containers and more specifically to an improved pressure resistant closure for a metallic container which is produced without heat.
Sealed metallic containers are used for the storage of many dangerous or reactive materials. When the contents of such a container are either extremely toxic or are radioactive, the seal of such a container must be air-tight and even pressure resistant. Most methods of making a pressure resistant seal on a metallic container involve the use of heat (e.g. welding). Such methods are unsuitable for sealing containers whose contents are flammable, explosive, or otherwise reactive.
Radioactive materials, such as sources or waste products, need to be handled remotely to minimize worker exposure. Extremely toxic materials, such as certain pesticides, herbicides, and chemical warfare agents, as well as biological agents and products of genetic engineering, also require remote handling.
While the present invention is useful for forming a pressure-resistant storage container for any type of material, the invention is especially useful as an overpack for a leaking container of hazardous or radioactive material. Overpacking containers in current use range from cardboard or plastic containers to metal 55 gallon drums. A more sophisticated overpack design is U.S. Pat. No. 4,100,860, which discloses an overpack of laminar construction having a layer of foamed polyurethane between its inner and outer shells.
One situation in which careful overpacking is especially important is in the storage of cesium and strontium capsules. Cesium and strontium are byproducts of the reprocessing of nuclear fuel. The surface dose rate of cesium capsules can be as high as 15,000 R/hr and strontium capsules can generate as much as 500-700 watts of thermal power. Because of the high dose rate and heat production of the elements, it is desirable to remove the elements from reprocessing waste before any storage of that waste. Current practice for both elements is to encapsulate them in stainless steel and store the capsules under water, which provides both shielding and cooling of the capsules.
If a capsule should develop a leak, the leaking capsule must be overpacked with an external cannister to prevent the release of radioactivity. Due to the high levels of radioactivity, such overpacking should be done remotely and beneath the surface of the water. The effects of heat and of radioactivity make the use of any organic sealing material impossible. Any water left inside the overpack will, by radiolysis, be broken down into hydrogen and oxygen, creating the possibility of fire or explosion so the overpack seal must be both air tight and pressure resistant.